Miniaturization proceeds in a mobile information terminal and a DSC (Digital Still Camera) and so on. With the miniaturization of the terminal, a battery is also miniaturized, and a case where a button cell of a small capacity is used is increasing. For this reason, a low consumed power amount is required even to an oscillation circuit which always operates inside the LSI (Large Scale Integration).
As a method of supplying power to such an oscillation circuit, a conventional technique is known in which a power supply voltage from a constant voltage circuit is lowered to a constant voltage, and the lowered voltage is supplied to the oscillation circuit. However, in such a conventional technique, as the voltage lowering width is increased, a resistance element having a higher resistance value is required. When the resistance element has the higher resistance value, a wider area is required for the resistance element in the LSI.
Therefore, the saving an occupation area in the LSI and the low consumed power amount are required.
In conjunction with the above, an oscillation circuit is disclosed in Patent Literature 1 (JP 2005-159786A). This oscillation circuit has as an object, to reduce consumption current. This oscillation circuit is provided with a functional section which restrains a supply voltage to an oscillation gate. This oscillation circuit restrains a voltage by using a threshold voltage of a transistor in a diode connection.
FIG. 1 is a circuit diagram showing the configuration of the oscillation circuit according to Patent Literature 1. Components of the oscillation circuit shown in FIG. 1 will be described. This oscillation circuit is provided with an oscillation gate circuit section 120, first and second amplitude restraining circuits 151 and 152, an amplitude amplifying circuit 109, an input side capacitor C101, an output side capacitor C102, a feedback resistance 103 and a resonator 104. The oscillation gate circuit section 120 is provided with a PMOS transistor 101 and an NMOS transistor 102. The first amplitude restraining circuit 151 is provided with a PMOS transistor 107. The first amplitude restraining circuit 151 is further provided with a diode 130, another PMOS transistor 132 and a first current source 134 and so on, but the description of these elements is omitted. The second amplitude restraining circuit 152 is provided with an NMOS transistor 108. The second amplitude restraining circuit 152 is further provided with a second diode 131, another NMOS transistor 133 and a second current source 135 and so on, but the description of these components is omitted. The amplitude amplifying circuit 109 is provided with a PMOS transistor 111 and an NMOS transistor 112.
The connection relation of the components of the oscillation circuit shown in FIG. 1 will be described. A source of the PMOS transistor 107 is connected with a power supply VCC. A gate and drain of the PMOS transistor 107 are connected with the source of the PMOS transistor 101 in common. The gate of the PMOS transistor 101 is connected with the gate of the NMOS transistor 102, one of the ends of the feedback resistance 103, one of the ends of the resonator 104, and one of the ends of the input side capacitor C101 in common. The drain of the PMOS transistor 101 is connected with the drain of the NMOS transistor 102, the other end of the feedback resistance 103, the other end of the resonator 104, one of the ends of the output side capacitor C102, a gate of the PMOS transistor 111 and a gate of the NMOS transistor 112 in common. A source of the NMOS transistor 102 is connected with the gate and drain of the NMOS transistor 108 in common. The source of the NMOS transistor 108 is grounded. The other end of the input side capacitor C101 is grounded. The other end of the output side capacitor C102 is grounded. The source of the PMOS transistor 111 is connected with the power supply VCC. The drain of the PMOS transistor 111 is connected with the drain of the NMOS transistor 112 and an output section CKOUT in common. The source of the NMOS transistor 112 is grounded.
An operation of the oscillation circuit shown in FIG. 1 will be described. In the first amplitude restraining circuit 151, the PMOS transistor 107 has the gate and the drain connected in the so-called “diode connection”. Therefore, the voltage of the drain of the PMOS transistor 107 is equal to a voltage lowered by the threshold voltage of the PMOS transistor 107 from the power supply voltage VCC. Hereinafter, the threshold voltage of the PMOS transistor 107 is referred to as Vt107.
In a same way, the NMOS transistor 108 is in a diode connection in the second amplitude restraining circuit 152. Therefore, the voltage of the drain of the NMOS transistor 108 is equal to a voltage arisen from the ground voltage by the threshold voltage of the NMOS transistor 108. Hereinafter, the threshold voltage of the NMOS transistor 108 is referred to as Vt108.
In the oscillation gate circuit section 120, the PMOS transistor 101 and the NMOS transistor 102 are connected in serial between the drain of the PMOS transistor 107 of the first amplitude restraining circuit 151 and the drain of the NMOS transistor 108 of the second amplitude restraining circuit 152. Therefore, an oscillation signal generated by the oscillation gate circuit section 120 has the amplitude between the voltage lower by threshold voltage Vt107 than power supply voltage VCC and the voltage higher by the threshold voltage Vt108 than the ground voltage.
Here, an output signal of the oscillation gate circuit section 120 is fed back to the input side through the feedback resistance 103 and the resonator 104 to be oscillated. It should be noted that when the oscillation signal of the oscillation gate circuit section 120 is generated, a charging operation and a discharging operation are performed in the input side capacitor C101 and the output side capacitor C102. The current which accompanies the charging/discharging operations is consumed in the oscillation circuit. In Patent Literature 1, the consumed power amount of the oscillation circuit is reduced by making the amplitude of the oscillation signal small.